1. Field of the Invention
The invention is directed to a solvent casting method for encapsulating water soluble reactive agents used in aqueous dentifrice compositions to prevent premature leakage into the dentifrice of the agents so as avoid interaction with other dentifrice ingredients.
2. The Prior Art
It has long been known to include fluoride containing compounds in dentifrices as anticaries agents, and it has been established that these compounds are effective to reduce the incidence of dental caries. Fluoride compounds which are deemed to be the most effective are sodium fluoride, sodium monoflurophosphate and stannous fluoride. The fluoride compounds are effective mainly due to the fluoride ions which improve the acid resistance of tooth enamel and accelerate recalcification of decayed teeth in their early stage when the decalcification has proceeded only slightly. The effect of improving the acid resistance of the enamel is believed to be due to the fact that the fluoride ions are incorporated into a crystal lattice of hydroxyapatite which is the main constituent of tooth enamel or, in other words, fluoride ions partially fluoridate hydroxyapatite and simultaneously repair the lattice irregularities.
The effectiveness of fluoride treatment in providing acid resistance is dependent upon the amount of fluoride ion which is available for deposition on the enamel being treated. It is, therefore, desirable to formulate dentifrice compositions which provide maximum fluoride ion availability in brushing solutions formed using the dentifrice.
It is known to the art, e.g., U.S. Pat. No. 5,045,305, that an effective way of depositing fluoride on teeth is to use a two-component rinse composition to deposit freshly precipitated calcium fluoride on teeth in which one rinse solution contains CaCl.sub.2 and the other contains fluoride ions in the form of NaF, the separate solutions being admixed immediately prior to introduction in the mouth, to effect interaction and rapid precipitation of CaF.sub.2.
U.S. Pat. No. 5,145,668 discloses a method of fluoridating teeth wherein there is mixed in the mouth a first solution containing a soluble calcium salt such as CaCl.sub.2 contained in a non-reactive vehicle and a second component containing a fluoride compound such as sodium fluorosilicate (Na.sub.2 SiF.sub.6) contained in a non-reactive vehicle, the mixing of the components resulting in the precipitation of calcium fluoride and its deposition on tooth surfaces.
Although the methods disclosed in U.S. Pat. No. 5,045,305 and U.S. Pat. No. 5,145,668 are effective means to achieve fluoridation, the separate solutions containing calcium and fluoride salts must be mixed daily which is a time consuming daily chore. As a result, it is very difficult for the potential beneficiaries of such therapy to faithfully adhere to the regimen. However, simply combining the calcium and fluoride salts into a single dentifrice formulation will not provide an effective means for fluoridation as the presence of the calcium salt removes soluble ionic fluoride from the dentifrice by forming insoluble and inactive calcium fluoride (CaF.sub.2) thereby reducing the anticariogenic effectiveness of the fluoride dentifrice.
Thus, there is a clear need to formulate a dentifrice product such as a toothpaste or gel utilizing a fluoride compound or other active compound wherein the ingredients used to prepare the dentifrice vehicle do not interact with each other such as in the aforementioned inactivation of fluoride ion so that optimum uptake of fluoride is accomplished when the dentifrice is applied to the teeth. Moreover, it is desirable to include water soluble reactive compounds such as calcium and fluoride salts in a single highly stable dentifrice form which is susceptible to conventional packaging and dispensing systems and which can be readily and effectively used by the consumer.
The dentifrice art discloses several means to isolate active ingredients from interaction with other ingredients present in the dentifrice. For example, U.S. Pat. Nos. 3,957,964; 3,929,988; 4,071,614; 4,220,552; 4,348,378; and, 4,376,762 disclose aqueous dentifrices containing ingredients such as flavors and dyes whereby such ingredients are encapsulated in rupturable, water-insoluble capsules so that the flavors and dyes are maintained substantially separate from other dentifrice ingredients during manufacture and storage, while subsequently being released when the dentifrice containing the encapsulated ingredients are applied topically to tooth surfaces, the mechanical agitation with a toothbrush rupturing the encapsulating shell whereby the encapsulated ingredient is released to the tooth surface. Materials from which the encapsulating shell is formed are diverse and include synthetic organic plastic materials such as phenol formaldehydes, vinyl chloride polymers, polyethylene, polypropylene, polyurethanes, ABS resins, waxes and cellulosic materials such as ethyl cellulose, butyl cellulose and nitrocellulose.
Although the water insoluble encapsulant materials of the prior art are effective to some degree to prevent interaction between water soluble reactive dentifrice ingredients, such materials were found to be inadequate to fully prevent interaction due to premature leakage of soluble reactive salts from the encapsulating material whereby, when encapsulated calcium salts were present in fluoride dentifrices, there resulted a significant loss of ionic fluoride and reduced levels of soluble fluoride availability in the dentifrice compositions stored for prolonged period of time.
U.S. Pat. No. 5,976,507 discloses one solution to the leakage problem encountered with water soluble reactive agents such as water soluble calcium salts encapsulated in alkyl cellulosic materials wherein the dentifrice in which the encapsulated reactive agent is present is prepared using an aqueous vehicle containing fluoride salts. In accordance with U.S. Pat. No. 5,976,507 one of the reactive agents is encapsulated by a spraying drying process wherein a water-insoluble, pressure rupturable shell is formed from a substantially water impermeable plasticized alkyl cellulose polymer which shell is rupturable during tooth brushing causing the encapsulated material to be released for interaction with the other active compounds present in the dentifrice composition. Although the plasticized alkyl cellulose was found effective in preventing of water soluble salts into the dentifrice, the encapsulated product was found to be too costly for wide spread commercial acceptance.